This invention relates Tacan distance measuring equipment in general and more particularly to apparatus for obtaining and developing signals which permit increased range accuracy in a Tacan system.
Tacan systems are widely used for navigation purposes both in commercial and military aircraft. In general, as is well known in the art, Tacan permits the determination of an aircraft's bearing to a Tacan station transmitter on the ground and also permits a measurement of distance from the aircraft to the Tacan station. The present invention is concerned only with the distance measuring equipment, (DME). In distance measuring, a pulse is transmitted by the aircraft and the ground based Tacan station responds thereto after a predetermined time delay which is known and fixed for each given Tacan system. The Tacan receiver is adapted to isolate the reply out of all the other signals being transmitted by the ground station and once its own reply has been isolated or acquired the range to the ground station can be determined since it will be directly proportional to the time between the request and the return transmission. (Taking into account, of course, the fixed time delay.)
The Tacan system is not a particularly precise instrument and is used primarily to obtain general indications of bearing and distance. Typically, the aircraft flies from one Tacan station to another and an error of even a mile or two is not significant.
In combat with helicopters, particularly in jungle areas, there is desire for the helicopter to fly as closely as possible to the tops of the trees to avoid detection and avoid anti-aircraft fire from the ground. Typically, in such situations, an existing clearing on the ground or one made for that purpose is used as a helicopter landing pad. It is necessary that the helicopter pilot be able to fly to the clearing and simply drop down. Since he is flying at a low altitude, he will not be able to spot the clearing from a distance. Because of this, there is need for a landing system to direct the pilot to the clearing with enough accuracy to then permit him to make a visual landing.
The landing system required could be a new system requiring additional navigation equipment in the aircraft. Such an approach would add weight and cost. Since each aircraft carries a Tacan receiver for other purposes, it is evident that if the Tacan system can be made more accurate it can be used in an effective landing system. It is then only necessary to set up a Tacan transmitter at the location of the landing site and use the conventional Tacan receiver in the aircraft along with a small amount of additional equipment to provide the landing system. Basically, two models of Tacan, one known as the ARN/52 and the other as the ARN/84, are presently in use. The accuracy of the older ARN/52 is relatively poor, the system having an accuracy of plus or minus 600 feet, plus or minus 0.2% of distance within the landing service volume of 0 to 50 miles. A workable landing system requires an accuracy which is an order to magnitude better.